In the past consolidation of thermoplastic sheets into solid, substantially flat panels of useful thicknesses for use as structural panels and the like has been achieved inside heated autoclaves under pressure or on heated platen presses. Such panels are useful in the aircraft industry because they have strength and weight characteristics superior to aluminum and at the same time can be machined by longstanding methods applicable to aluminum which has been used extensively in the manufacture of aircraft.
The autoclave process is slow and inefficient in its use of energy. The sheets are required to be confined for several hours in the sealed environment of the autoclave where monitoring of the process is difficult. Not only is this process of placement of the multiple, stacked sheets in the autoclave time-consuming, but if there is poor bonding or some mistake in the process, it is not discovered for hours, after removal of the sheets from the autoclave. As a result there is a loss of the investment in autoclave time, manpower and auxiliary materials.
As for laminated panels formed on a press, the platen size limits the size of panels which can be produced. Internal dimensions of autoclaves also limit the size of panels which can be produced in them. When large (for example, 24 feet by 8 feet) single panels are required, as for aircraft wing skins, acquisition costs for presses as well as for autoclaves can be prohibitive.
The foregoing deficiencies of the current practices leave much to be desired in filling the needs of high productivity and automation, especially in the aircraft industry. Because of the ever-growing use of thermoplastic panels in structural applications to airplane manufacture, a faster, less expensive and more reliable and efficient means for forming these panels especially in large sizes has become important.